The present invention relates in general to detecting objects and/or areas. More particularly, the invention provides a method and system for signal correlation. Merely by way of example, the invention is described as it applies to correlating two signals, but it should be recognized that the invention has a broader range of applicability.
In radio astronomy, for example, the measurement of the cross correlation between two signals is required. A measurement configuration usually comprises of two spatially separated antennas pointing in the same direction to receive the microwave energy radiated by a selected radio star. The received microwave energy from the radio star is expected to be much weaker than the microwave energy received from the terrestrial thermal surroundings of the antennas.
There are a number of conventional systems and methods for measuring the cross correlation of signals. For example, a correlation receiver may function as a radiometer. The system may include two bandpass limiters that are located between the outputs of two radio receivers and the inputs to a multiplier. The bandpass limiters reduce the amplitude modulation at the inputs of the multiplier due to the fluctuation in the amplification gain of the radio receivers.
Another conventional method uses a quadrature hybrid to transform two independent thermal signals into two other signals that are coherent with respect to each other. Yet another method uses a 180-degree hybrid to transform two independent thermal signals into other signals to be correlated. Depending on which method is applied, the output of the correlator is proportional to the sum or the difference of power in the two independent thermal signals.
In the conventional systems and methods, the antenna and other microwave components located in front of the correlator contributes a small amount of noise because of the finite temperature of these components. In radio astronomy applications, the noise power contributed by the antenna and microwave components in front of a correlator could be orders of magnitude higher than the external signal received by the antenna. To reduce the contribution of noise from front-end components, radio astronomers usually use spatially separated antennas and low noise radio receivers to make correlation measurements.
For example, a method of measuring both the amplitude and the phase of the cross correlation of two signals uses two multipliers and a quadrature network. The result is a complex cross correlator. But the fluctuation in the amplification gain of the correlative receivers is usually not compensated. As another example, a digital cross correlator can acquire and track spread spectrum communication signals. This type of correlator is designed to correlate a known binary sequence with an unknown signal embedded in noise.
As discussed above, a number of types of correlation receivers have been developed in prior art techniques. When the weak signals to be correlated are themselves sums of plurality of weaker signals, the task to determine the coherence between them can be quite challenging. Conventional correlation receiver usually cannot effectively extract and separate the correlation properties among weak and complex signals.
Hence it is highly desirable to improve cross-correlation techniques.